The present invention pertains to the assembly of roller cone type rock bits. Such bits have enjoyed widespread use in the petroleum industry for many years. Although many improvements have been made in this type of bit to prolong its life, the bits remain primarily a "throw away" item today. That is to say, once the bit, or any portion thereof, is damaged or worn to the point that the bit can no longer function effectively, the entire bit is typically discarded or scrapped and replaced with a new bit, which is quite expensive.
Typically, the wear or damage which causes the bit to stop functioning effectively occurs in or near the roller cone assemblies, e.g. on the working surfaces of the cones themselves and/or in their bearings, as opposed to the upper portion of the bit. Since the upper portion contains the tool joint, specialized formations for lubricant delivery, and the bulk of the material of the bit body, it represents a considerable portion of the overall cost of a new bit. Thus, if the upper portion can be salvaged and reused, a rebuilt bit can be provided at a considerable saving over the cost of a new bit.
U.S. Pat. Nos. 3,850,250 and 4,158,973 suggest methods whereby the legs of a bit with cones mounted thereon are welded to a full annular body portion. The juncture surfaces thus welded are plane horizontal surfaces extending perpendicular to the bit centerline. Said patents also suggest the use of high energy beam welding techniques, such as electron beam welding. Such welding techniques are relatively fast, and minimize heating of the parts being welded. However, they can create other problems. For example, where a weld is formed by a high energy beam, such as an electron beam, the high energy beam emerges from the far side of the joint and strikes any object in its path. The aforementioned horizontal orientation of the surfaces to be joined, as suggested by said prior patents, allows the high energy beam to strike structures which can be damaged thereby, e.g. the tungsten carbide inserts which are carried on the roller cones.
Another group of problems revolves around the fact that the high energy beam tends to rapidly and forcefully displace material in the direction in which it is directed along the joint. Thus, as the weld is formed, a small quantity of molten metal from the parts being joined will actually be carried outwardly from the far side of the joint by the beam and deposited upon any structure which lies in the beam's path. This material, should it land on one of the aforementioned tungsten carbide cutting inserts, would be quite undesirable. The material might also be deposited on one of the openings, bores or passageways which extend through the bit for various purposes, thereby blocking them. The rapid, high energy displacement of material can also cause a problem known as "out-gassing." This can occur when the beam passes a bore or other cavity in the joint area. Gases from such cavity can be "blown" outwardly by the beam, creating bubble-like areas in the finished joint. A similar and related problem is that the material displaced by the beam can partially or even completely plug such cavities, e.g. the lubricant supply channels which extend along the bit legs crossing the weld joint area.
All of the above problems can occur with the joint configurations suggested in the aforementioned prior patents. Efforts to ameliorate at least some of these problems by placing temporary blocking structures along the downstream edges of the joints during welding have not been entirely satisfactory.
Still another problem with these joint configurations is related to the fact that the roller cone assembly must be substantially fully assembled, including the elastomeric seal which seals between the rotary cone and the stationary base, before the base is welded to the main body member of the bit. Even though high energy beam welding techniques tend to minimize heating of the parts being joined, they necessarily cause a significant amount of heating, and with the aforementioned joint configurations, the area of the elastomeric seal would tend to be heated sufficiently to cause concern over the integrity of the seal.